The present invention relates to a process for preparing a molding powder of polytetrafluoroethylene (hereinafter referred to as "PTFE"), and more particularly to an improved process for preparing agglomerated PTFE granules containing a filler material.
The PTFE molding powders are obtained by grinding granules made by suspension polymerization into finely-divided particles, and have been fabricated by compression molding or ram extrusion. The particle size of the finely-divided particles ranges from about 5 .mu.m. minimum to about 1,000 .mu.m. maximum, but is usually less than about 100 .mu.m. In order to improve the abrasion resistance and hardness of the molded articles of PTFE, such molding powders are uniformly incorporated with one or more than two kinds of filler materials, e.g. a glass bead, powder or fiber, carbon, graphite, molybdenum disulfide, bronze powder or various heat resisting synthetic resin powders such as polyimide, polyamideimide and polyphenyleneoxide, which are called and employed as filled PTFE molding powder.
With increasing demand for automatic molding in recent years, an agglomerated granular type of filled PTFE molding powder having improved handling characteristics, particularly improved flowability and high apparent density has been prepared and employed.
Processes for agglomeration are mainly classified into a dry process in which water is not employed and a wet process in which water is employed. A typical one of the dry processes is a process described in U.S. Pat. No. 3,781,258 in which an intimate mixture of PTFE and a filler is wetted with an organic liquid capable of wetting the PTFE particles such as carbon tetrachloride, acetone, trichloroethylene or a fluorinated and chlorinated hydrocarbon and is then subjected to the mechanical force such as agitation. In a wet process, a mixture of the PTFE particles, a filler and a water-immiscible organic liquid as used in the above-mentioned dry process is agitated in a considerable amount of water. Such a wet process requires steps for the separation and drying of water after the agitation treatment, but is superior to a dry process in that automatization of the manufacturing process is relatively easy.
However, the wet process has the disadvantage that in case of using a hydrophilic filler such as glass fiber, the filler is easy to migrate to water phase due to good affinity with water and is hard to uniformly admix with the PTFE particles. As a result, all amount of the filler is not incorporated into the agglomerated granules, and a part of it remains in water. This phenomenon is called separation of filler. In order to solve this problem, there are proposed a process in which a filler is previously subjected to surface treatment to lower the surface activity of the filler and make its surface hydrophobic, thus bringing the surface activity close to that of PTFE, and thereafter such a filler is utilized in agglomeration in an aqueous medium; and a process in which such a surface-treated filler is not utilized, but an additive that affords similar function to the filler is added to an aqueous medium upon agitation for agglomeration. Those surface treating agents or additives are (a) silanes having amino functional groups and/or soluble silicones as disclosed in U.S. Pat. No. 3,929,721, (b) C.sub.12 to C.sub.20 aliphatic carboxylic acids as disclosed in Examined Japanese Patent Publication No. 37576/1973, (c) chromium complexes of aliphatic carboxylic acids as disclosed in Examined Japanese Patent Publication No. 37576/1973, and (d) silicones as disclosed in Unexamined Japanese Patent Publication No. 139660/1978. It is also known from (e) Unexamined Japanese Patent Publication No. 121417/1976 to coat hydrophilic filler particles with PTFE.
However, these known processes have still problems to be solved. That is to say, it is hard to achieve shortening the steps of the process, since in all these processes a uniform mixture of the PTFE particles and a filler must have been prepared previously and thereafter subjected to agitation for agglomeration. Moreover, some kinds of fillers inevitably accompanies a problem to cause harm to the working environment due to dust formation. Further, the above-mentioned processes (a), (b) and (d) are insufficient in the effect of preventing the separation of filler, and the process (c) causes coloration on the molded articles after sintering. The process (e) is relatively advantageous for preventing the separation of filler, but the flowability of the agglomerated granules is insufficient because PTFE fine powder, i.e. dispersion grade PTFE powder which is obtained by polymerization of tetrafluoroethylene in the presence of a dispersing agent, is used.
It is an object of the present invention to provide a process for preparing a filled PTFE molding powder.
A further object of the invention is to provide an improved process for preparing agglomerated PTFE granules containing a filler in shortened process steps without causing the separation of the filler.
A still further object of the invention is to provide a filled PTFE molding powder having a good flowability and capable of giving sintered moldings having an improved surface smoothness.
These and other objects of the present invention will become apparent from the description hereinafter.